python jpeg_codec.py -f dog.jpg

The encoding process consists of several steps:

The representation of the colors in the image is converted from RGB to a color space called YCbCr. Is has three components Y, Cb and Cr. The Y component represents the brightness of a pixel, and the Cb and Cr components represent the chrominance (split into blue and red components). The YCbCr color space conversion allows greater compression without a significant effect on perceptual image quality off the image.

The resolution of the chroma data is reduced, usually by a factor of 2 or 3 (downsampling). This reflects the fact that the humans can see considerably more fine detail in the brightness of an image (the Y component) than in the hue and color saturation of an image (Cb and Cr components). The ratios at which the downsampling is ordinarily done for JPEG images are:

• 4:4:4 : No downsampling
• 4:2:2 : Reduction by a factor of 2 in the horizontal direction
• 4:2:0 : Reduction by a factor of 2 in both the horizontal and vertical directions

The image is split into blocks of 8x8 pixels, and for each block, each of the Y, Cb and Cr data undergoes the discrete cosine transform (DCT).

The amplitudes of the frequency components are quantized.

TODO: Write